Method for producing angular, stainless shot-blasting abrasives based on an fe-cr-c alloy

ABSTRACT

The invention relates to a process for producing a corrosion-resistant blasting agent (&gt;60 HRC) with sharp edges based on an Fe—Cr—C alloy. A granulate consisting of an iron-chromium-carbon alloy is hardened in said process to &gt;60 HRC by subjecting it to a heat treatment at above 900° Celsius under a reducing atmosphere. An oxide-free, hard material is obtained in this way that can be crushed into grains with sharp edges. The result is a blasting agent with excellent properties for the surface treatment of workpieces consisting of corrosion-resistant materials such as, for example stainless steel, nonferrous metals and natural stone.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of German Application No.100 02 738.5, filed on Jan. 22, 2000. Applicants also claim priorityunder 35 U.S.C. §365 of PCT/EP01/00252, filed on Jan. 11, 2001. Theinternational application under PCT article 21(2) was not published inEnglish.

The invention relates to a process for producing grains of blastingagents consisting of rustproof cast stainless steel, in which agranulate is produced first from a the melt of an iron-chromium-carbonalloy capable of being hardened. Said alloy is then passed through aheat treatment at >900° C. for hardening it, and subsequently crushed tograins with sharp edges.

For the blasting treatment of workpieces consisting of stainlessmaterials it is necessary to employ stainless blasting agents as wellbecause rusting blasting agents such as steel shot or steel gravel leaveiron-containing residues on the surface of the workpiece. Due to theoxidation of adhering iron residues, undesirable stains of rust thenappear within a very short time. In addition to nonmetallic, mostlymineral blasting agents such as, for example electro-corundum, siliconcarbide or glass, stainless metallic blasting agents are known as well.To be mentioned in this connection is stain steel blasting shotconsisting of corrosion-resisting steel alloys. Such material offers anumber of advantages vis-à-vis the mineral blasting agents. For example,a substantially prolonged useful life can be achieved with the metallicblasting grains on the usual blasting equipment because the stainlesssteel, by virtue of its greater ductility, is crushed in the course ofthe blasting treatment to a substantially lesser degree. Stainless steelblasting agents have been successfully employed especially when used inblasting plants equipped with centrifugal wheels because of their goodwear properties conditioned by its high impact strength.

Two categories of blasting agents consisting of non-corrosive cast stainless are known. These are granulates consisting of spherical grains thatare made of steel materials with medium hardness (<45 HRC), on the onehand. As disclosed in JP 61 257 775, on the other hand, grains havingsharp edges and consisting of hardened cast chromium iron (>60 HRC) areemployed because enhanced abrasion properties can be achieved with suchgrains.

Substantially higher manufacturing expenditure and additional processsteps are required in the manufacture of the hardened granulate withsharp edges vis-à-vis the blasting agent grains of the first category.In the manufacturing process according to JP 61 257 775, a granulateconsisting of substantially round grains is produced first based on amelt of a chromium-cast iron alloy that is capable of hardening. Saidgranulate is hardened by quenching it at 1000° C. to 1100° C. in water,following a thermal treatment. The grains are subsequently crushed, sothat a material with sharp edges is obtained.

The drawback of said method is that the undesirable oxidation of thematerial is favored by quenching the hot steel in water, said steelhaving a temperature of more than 1000° C. Furthermore, the achievablecooling rate is highly limited when water is used (steam phase).However, effective quenching is absolutely required in order to obtain amaterial that is as brittle as possible. This is the precondition thathas to be satisfied so that the grains can be crushed later in such away that the desired granulate with sharp edges is produced.

The present invention is accordingly based on the problem of providing aprocess for producing corrosion-proof blasting agents, in whichoxidation of the granulate can be excluded during and after the finalheat treatment, and in which the brittleness of the material achieved inthe hardening step is adequately high for crushing the blasting grainswith simple means to obtain a granulate with sharp edges.

Said problem is solved in conjunction with a production process of thetype specified above in that the thermal treatment is carried outat >900° Celsius in a reducing atmosphere, and in that a reducing gas orgas mixture is used in the subsequent cooling step as well.

Owing to the fact that the granulate is exclusively subjected to areducing atmosphere in the hardening phase, the advantage ensues thatundesirable oxidation of the material is reliably avoided.

The reducing atmosphere usefully is a gas mixture that contains hydrogenand nitrogen. It has been found in practical life that a gas mixturecontaining from 60% to 80% hydrogen and from 20% to 40% nitrogen isparticularly suited for the process as defined by the invention. Thebest results were achieved with 70% hydrogen and 30% nitrogen.

Special process steps have to be maintained in order to produce blastingagents from a cast iron-chromium alloy. The use of aniron-chromium-carbon alloy with at least 2% carbon and at least 30%chromium results in a material that can be hardened in acorrosion-resistant manner, whereby degrees of hardening of >60 HRC canbe achieved without problems. A material is obtained in this way that ischaracterized by high resistance to oxidation and excellent resistanceto wear. The use of the alloy specified above is therefore especiallyuseful in the process as defined by the invention because it offers thecombination of a material that can be hardened in a good way and isresistant to corrosion at the same time.

For crushing the hardened granulate it is useful if a pulsed mill isemployed. A tubular oscillating mill is particularly suited forproducing from the hardened starting material the desired granulate withsharp edges.

If the blasting agent is employed for the surface treatment of metallicworkpieces it is useful if it is present classified according to grainsize. The production process as defined by the invention can be followedfor such a treatment by an additional process step downstream forfractionating the grains. The adjustment of the desired grain mixture isachieved with an extra process step.

The process as defined by the invention is explained in greater detailin the following with the help of the drawing.

The drawing shows a flow diagram of the production process, whereby theupper part of the drawing comprises the process steps for producing thestarting granulate, whereas the lower part shows the process stepshardening, crushing and classifying.

The starting material for the blasting agent is steel scrap that is fedinto the production process from a shot storage bin 1. Carbon in theform of the graphite 2 and the chromium 3 is added from suitable supplyreservoirs for adjusting the desired alloy. The raw material mixture issubsequently melted to an alloy in a melting furnace 4. Said alloycontains 2.0% carbon and 30% to 32% chromium.

The melt passes through an atomizing device 5 at a temperature in excessof 1420° Celsius, whereby a granulate with a wide spectrum of differentgrain sizes is produced. The atomized droplets of the metal melt arequenched in a water bath, so that a solid granulate is collected on thebottom of a granulating trough 6.

The granulate is withdrawn from the trough via an outlet 7 and passesthrough the process steps 8, “dripping off”, and 9, “drying”. Afterpassing through a cooling stage 10, the starting material for thecorrosion-resistant cast chromium alloy is present.

The starting material is now charged in a furnace 11, in which it isannealed at low pressure and at over 900° Celsius in an atmosphere 13 ofhydrogen and nitrogen, and is cooled subsequently thereto. Thereafter,it is conveyed into a supply container 12. Annealing of the granulateat >900° C. causes separation of secondary carbides from the alloy-richmatrix, which changes the composition of the matrix. Conversion intomartensite is possible only by separating the secondary carbides, whichthen leads to an increase in the hardness to >60 HRC when the granulateis cooled down from temperature of >900° C.

From the container 12 the granulate is fed into the crusher 15 by abucket mechanism 14. The crusher 15 is preferably realized in the formof a tubular oscillating mill, and crushes the hardened, brittlegranulate to fractured bodies with sharp edges. By employing such pulsedmills it is possible in a particularly good way to break up thematerial, which is under strong inner tension, into fragments with sharpedges. The grain mixture produced during crushing has a widedistribution of different grain sizes. For classifying purposes, thegrain mixture now passes through a screening plant 16. The excessivelycoarse, oversized grains 17 are recycled into the crusher. Theexcessively fine, undersized grains 18 are removed from the process atthis point and melted down in the melting furnace 4. The good grains 19with a diameter of between 0.1 and 0.8 mm are either stored in a storagebin 20 or charged in another screening plant 21 for fine classifying.Blasting agents each having different grain sizes are stored in thestorage bins 22, 23 and 24, until they are removed for shipment to thefinal consumers.

What is claimed is:
 1. A process for producing blasting agent grainsfrom non-corrosive cast stainless steel comprising the steps of: (a)producing a granulate from a melt of an iron-chromium-carbon alloycapable of hardening and containing at least 2% carbon and at least 30%chromium; (b) passing said granulate through a thermal treatment in areducing atmosphere at >900° Celsius for hardening; (c) subsequentlycrushing said granulate into grains with sharp edges; and (d) coolingsaid grains in a reducing atmosphere.
 2. The process according to claim1, wherein the reducing atmosphere is a gas mixture containing hydrogenand nitrogen.
 3. The process according to claim 1, wherein the granulateis crushed by means of a pulsed mill.
 4. The process according to claim3 wherein the pulsed mill is a tubular oscillating mill.
 5. The processaccording to claim 1 further comprising the step of carrying out grainfractioning to set various grain mixtures after cooling said grains. 6.A process for producing blasting agent grains from non-corrosive caststainless steel comprising the steps of: (a) producing a granulate froma melt of an iron-chromium-carbon alloy capable of hardening; (b)passing said granulate through a thermal treatment in a reducingatmosphere at >900° Celsius for hardening; (c) subsequently crushingsaid granulate into grains with sharp edges; and (d) cooling said grainsin a reducing atmosphere comprising from 60% to 80% hydrogen and from20% to 40% nitrogen.